1. Field of the Invention
The present invention relates generally to implantable orthopedic prostheses for replacing human skeletal joints, and relates more particularly to humeral components of implantable orthopedic shoulder prostheses for reconstructing a shoulder joint in a human.
2. Background of the Related Art
Implantable orthopedic prostheses, in one form, comprise man-made replacements for the ends and articulating surfaces of the bones of the skeleton. Such prostheses are implanted to repair or reconstruct all or part of an articulating skeletal joint that is functioning abnormally due to disease, trauma, or congenital defect. Among the various articulating skeletal joints of the human body that are eligible to be fitted with implantable orthopedic prostheses, the hip, knee and shoulder joints are most often treated with such prostheses. The performance of each of these joints has an important effect on quality of life. The hip and knee joints play a critical role in ambulation and the shoulder joint plays a critical role in manual dexterity, resulting in great demand for surgical correction of abnormalities of these joints.
As used herein, the words proximal and distal are terms of reference that indicate a particular portion of a bone or prosthesis component according to the relative disposition of the natural bone or implanted prosthesis. Proximal indicates that portion of a component nearest the torso, whereas distal indicates that portion of a component farthest from the torso. Directional terms of reference used herein include superior, inferior, anterior, posterior, medial and lateral, which are used according to their commonly understood anatomical meanings. More particularly, with regard to a person in a standing position, superior means upward, inferior means downward, anterior means forward, posterior means rearward, medial means inwardly from the side toward the center of the body, and lateral means outwardly from the center of the body toward the side.
The human shoulder joint involves two bones: the humerus and the scapula, each having a smooth articulation surface arranged for articulation against an adjacent articulation surface of the other bone. The humerus includes at its proximal extremity a head having a convex, nearly hemi-spherically contoured articulation surface. With the arm in its natural position hanging adjacent the torso, the axis of the hemi-spherically contoured head is directed generally superiorly, medially, and posteriorly. The axis of the head forms an angle of about 45.degree. to the superior-inferior axis, and an angle of about 35.degree. to the medial-lateral axis. These angles are typical and can vary considerably between individuals. The articulating hemispherical surface of the head is covered with smooth cartilage. The scapula, in pertinent part, includes a bony formation known as the scapula head located on the superior lateral border. A glenoid cavity is located between the acromion process and the coracoid process of the scapula head. The glenoid cavity is a shallow, pear shaped, concave surface opening laterally and anteriorly, and elongated in the superior-inferior direction. The glenoid cavity is covered with smooth cartilage against which the head of the humerus articulates.
Shoulder prostheses typically include a stem to be implanted within the proximal medullary canal of a humerus, the medullary canal having been exposed by surgical resection of the natural humeral head. The stem includes a head affixed to the proximal end of the stem and having a spherical surface to engage and articulate against the glenoid cavity of the scapula. Prior humeral prostheses have been assembled from modular components comprising separate stem and head members. Connection between the head and stem usually is accomplished with mating male and female conical tapers, also known as Morse tapers. The mating conical tapers provide a firm and durable connection, yet permit the stem and head components to be disassembled from each other, if desired. Sometimes, a prosthetic glenoid component is also implanted against the scapula to provide a prosthetic articulating surface against which the head can articulate. Typically, stems are provided in a range of sizes involving variations in length and diameter, and heads are provided in a range of sizes involving variations in diameter and radius of curvature of the spherical articulating surface. Each of the various stems and heads employs commonly dimensioned conical taper surfaces to ensure interchangeability and modularity. The male conical taper can be disposed on the humeral stem, with the female conical taper disposed on the undersurface of the head, or vice versa.
Typical prior modular humeral shoulder stem prostheses have included a stem having a distal stem portion and a proximal body. The distal stem portion is generally cylindrical and elongate, whereas the proximal body is generally trapezoidal to approximate the shape of the proximal medullary canal of the humerus. The proximal body can have a planar portion disposed generally transversely to the axis of the natural humeral head, at an angle of about 45.degree. to the longitudinal axis of the distal stem portion. A male conical taper can extend perpendicularly from the planar portion. Conversely, a female conical taper can be depressed in the planar portion.
Typical prior modular humeral shoulder head prostheses have included a generally planar undersurface. A female conical taper can lie depressed in the planar undersurface of the head for receiving the male conical taper of the humeral stem. When so received, the planar undersurface of the head lies adjacent the planar portion of the proximal body of the humeral stem. Conversely, the head prosthesis can include a male conical taper extending perpendicularly from the generally planar undersurface for mating with a humeral stem in which the proximal body includes a female conical taper. The articulating surface of the head, disposed opposite the planar undersurface, typically comprises a portion of a sphere subtending an angle less than 180.degree. and intersecting the planar undersurface circumferentially. Usually, the conical taper connectors are coaxial with the axis of symmetry of the spherical articulating surface, and centered with respect to the planar undersurface of the head.
Prior humeral shoulder prosthesis configured as described above are generally satisfactory for most human patients. In some patients, however, certain disadvantages of the above-described configuration have been noted. Firstly, the superior edge of the head prosthesis may extend beyond the proximal humerus, creating undue stress on the rotator cuff muscles. Secondly, the inferior edge of the head prosthesis may contact the glenoid, or glenoid prosthesis, during some portions of the range of motion of the shoulder joint. Such contact happens especially when the arm is close to the torso, creating an "edge contact" between the head prosthesis and glenoid which accelerates wear of the articulating components. Thirdly, some patients have an anatomical condition in which the posterior cortical bone at the superior end of the humerus extends excessively posteriorly of the center of the medulary canal. In such patients, since the location of the head is dictated by the location of the stem in the medullary canal, the head of the humeral prosthesis may be located too far anteriorly to cover the posterior cortical bone.
It would be desirable to provide a modular, humeral shoulder prosthesis that can alleviate each of the mentioned disadvantages of prior prostheses, thereby permitting a wider range of anatomical variations to be accommodated. The present invention, particular embodiments of which are shown and described below, provides these and other desirable advantages.